Your odds of getting in are decent. The question is really: getting in... to what? If it's just getting a job as a non-union cashier for $8 an hour, there are always jobs there. Not because it's a growth industry but because it's a lousy, low paying job, generally speaking.
Stores can't hire people with much talent for such a low wage and, well, they quit. Or just one day don't show up. Or rip off whatever cash was in the register and head straight to Mexico. So replacement troops are always needed.
Getting into the cashier's union, however.... the land of $20/ hour labor plus benefits... is tough. Most unions protect their own members and do what they can so that the existing union cashiers are taken care of. They know that their power is shrinking and, with a more constrained labor supply, they have more control over their efforts, strike funds, and other negotiating issues.
Sure, they'd love it if the industry was growing… but between the slow automation of the process and sites like Amazon.com, the live check-out cashier world is slowly dinosauring. Most of the ones still working today are already fossils.